This invention relates in general to the field of suppression of image noise in communications, and in particular to suppression of image noise during frequency downconversion in superheterodyne communications receivers.
A well-known occurrence in superheterodyne receivers is that the front end low-noise amplifier in such systems will generate thermal noise at the image frequency and that during the downconversion process the image noise will "fold over" onto the thermal noise at the desired receiver frequency. To avoid the associated degradation in system sensitivity, 15-20 decibels (dB) of image noise rejection is required prior to downconversion.
There are two general methods for providing such image rejection in communications receivers. The first uses a bandpass filter (image filter) centered at the desired receive frequency and connected between a low noise amplifier and a downconversion mixer. The bandpass filter is designed to provide 15-20 dB of noise suppression at the image frequency while passing the desired receive frequency (RF). For receiver applications where the intermediate frequency (IF) is very low relative to the RF frequency, the required Q of the image filter can be very high since the percentage difference between the RF and the image frequencies is very small (i.e., the local oscillator (LO) frequency is very close to the RF). High Q filters are typically realized using air dielectric cavity filter configurations. Major drawbacks to this method are that cavity filters are physically large, must be aligned prior to installation into a module, and require input/output transitions between the cavity transmission medium (coaxial, waveguide, etc.) and the planar transmission medium (typically microstrip).
The second method for providing image rejection incorporates a conventional image reject mixer whose topology is designed to downconvert the LO frequency plus the IF and the LO frequency minus the IF sidebands into separate IF output ports. However, considerable mixer complexity and development risk results from this method, especially at the higher microwave frequencies. The mixers must be well matched and the phase relationships well maintained in order to achieve adequate image suppression. In addition, the required local oscillator power for this method is 3 dB higher than that required for a comparable non-image rejection mixer.
Thus, what is needed is a relatively simple, efficient, and easily maintained method and apparatus for image suppression in communications receivers which is implementable with any standard mixer and which is smaller than conventional cavity filters and does not require filter input/output transitions. It would be additionally desirable if such a method and apparatus would provide selectable bandwidth capability.